Instagram, which just beat out Zynga's FarmVille 2 for the top of AppData's much-watched chart of monthly active users.

Given Instagram's runaway growth this year, it wouldn't be surprising that Instagram hit the top of the charts—except for the recent fuss over Instagram's terms of service, which some users misinterpreted as authorizing Instagram to sell their photos to advertisers.

There's also the mysterious drop in Instagram's daily active users from the same source, AppData, that just placed them at the top on a monthly basis. Some reporters suggested that the drop, which happened around Christmas Day, was due to a user revolt.

Instagram's daily numbers dropped 25 percent last week, and have continued to drop, even as its monthly numbers have surged upwards.

Facebook has said that AppData's daily numbers are "inaccurate," but hasn't offered more details on why, which is a shame, since Facebook owns Instagram and could share traffic data from its own site and Instagram to explain the discrepancy.

"We definitely had hiccups on [Let's Date] when we first moved to native iOS6 Facebook," Simitzis told us on Twitter.

Instagram recently updated its app to take advantage of a feature in iOS, Apple's operating system for mobile devices, to connect more directly to Facebook.

Downloads of Instagram on Apple's App Store and the Google Play store for Android apps have remainedsteady. In fact, Instagram saw a recent surge in its download rank on the App Store right around the same time its daily AppData numbers started dropping.

AppData only measures usage of Instagram when users choose to connect it to Facebook, so it doesn't capture all usage of the app. Inside Network, the operator of AppData, hasn't responded to our inquiry about the discrepancy.